The objective of this proposal is to further investigate the inhibitory nature that the hippocampal formation (HPCF) has on the luteinizing hormone (LH) secretory apparatus. The first experiments proposed set out to determine whether the inhibition arises from either, one of the HPCF subdivisions, i.e. dorsal hippocampus, ventral hippocampus and ventral subiculum or from the entire HPCF. To this end each one of the subdivisions will be selectively stimulated (electrochemical stimulation) on the day of proestrus and the effect on plasma LH that day and ovulation on the next day determined. Other animals will undergo HPCF stimulation following destruction of the ventral subiculum or the medial corticohypothalamic tract. Plasma LH determination and examination of ovulation will also be done on these animals. Other experiments set out to examine the effects of HPCF stimulation on the content, and turnover of noradrenaline (NA), dopamine (DA), serotonin (5-HT) and acetylcholine (ACH) in the medial preoptic area (MPOA), arcuate nucleus (ARC) and median eminence (ME) as well as plasma LH levels on the day of proestrus. Six hours after stimulation the rats will be killed, a blood sample collected and the brains processed for neurotransmitter determination using the Palkovits (1973) technique for dissecting out the brain nuclei. A third set of experiments sets out to determine the effect of HPCF stimulation during the day (1100-2100 hrs) of proestrus on the release of NA, DA, 5-HT and Ach in the MPOA and ARC as well as plasma LH levels in freely moving awake rats outfitted with permanent electrodes in the HPCF, push-pull cannulas in the MPOA and ARC and a chronic i.v. cannula in the jugular vein. During the day of proestrus the rats will undergo HPCF stimulation along with perfusion of the MPOA and ARC for neurotransmitter determinaton and blood sampling through the cannula for LH determination. On the next day the uterine tubes will be examined for the presence of ova. The results obtained from these experiments will aid in the better understanding of brain mechanisms that are inhibitory to LH release as well as in a better understanding of neuroendocrinological mechanisms in general.